Polyester resins have found widespread use in varied applications. Polyesters such as polyethylene terephthalate are used in photographic film, in magnetic tape, in fibers, and in food and beverage containers. Various methods have been disclosed for the depolymerization of such resins into their component monomers, such as ethylene glycol and terephthalic acid or derivatives thereof, so that they can be reused.
Some of these methods are described in such patents as U.S. Pat. Nos. 3,037,050, 3,321,510, 3,884,850, 3,907,868, 4,163,860, 4,578,502, 4,620,032, 4,876,378 and 5,095,145, and in European Published Patent Application 0 484 963 published May 13, 1992.
A particularly useful technique for recovering polyester monomers is described in Naujokas et al. U.S. Pat. No. 5,051,528. This patent describes a process of recovering ethylene glycol and dimethyl terephthalate from polyethylene terephthalate by dissolving the polyester resin in oligomers of the same monomers as are present in the polyester by passing super-heated methanol through the solution to depolymerize the polyester resin. Methanol, component monomers and impurities are then separated in a series of distillations. For ease of operation, the process of the '528 patent, preferably is performed at ambient pressure.
Gamble et al. U.S. Pat. No. 5,298,530 improves the process of the '528 patent by adding polyester resin to the system in a dissolver, where it is combined with reactor melt before the combination is transferred to the reactor for contact with super-heated methanol. In the reactor, polymers and oligomers are further depolymerized into component glycol and ester monomers, which are then recovered. This permits operating the dissolver at ambient pressure while operating the reactor at elevated pressure.
Toot et al. U.S. Pat. No. 5,414,022, optimizes the process of the '530 patent.
The processes described in this series of patents have numerous advantages. These include low cost, high efficiency, the ability to operate at relatively low pressure and the ability to use a variety of forms of polyester of varying degrees of cleanliness and purity.
However, when the process of the '528 patent is operated at ambient pressure, and the output of the reactor is cooled to commence separation of monomers, a three phase system is formed comprising Gaseous methanol, liquid ethylene glycol and solid dimethyl terephthalate. The low solubility of the dimethyl terephthalate in ethylene glycol results in its depositing on the surface of the cooling apparatus. This results in poor heat transfer, inefficient operation and the need for frequent cleaning.
The dissolver described in the '530 and '022 patents was developed, in part, to overcome this problem. The presence of the dissolver, which operates at ambient pressure, permits the reactor and the cooling apparatus to be operated at a sufficiently elevated pressure that a two phase system is maintained. In such a system the dimethyl terephthalate stays in solution until it is separated in a subsequent operation. Thus, it does not foul the cooling apparatus. Nevertheless, operating part of the recovery system at elevated pressure adds inconvenience and cost. Thus, it would be desirable to modify the apparatus and process so that the reactor and the recovery apparatus can be operated at ambient pressure, while minimizing fouling of the cooling apparatus.